Hierarchical audio coding refers to organizing code streams of audio coding in a manner of hierarchy, generally it is to divide into a core layer and several extended layers. A decoder may only decode the code streams of lower-layer (such as the core layer) in the absence of code streams of higher-layer (such as the extended layer) coding, and the more the decoded layers are, the higher the tone quality is.
The hierarchical coding technology is of very significant utility value for the communication network. On one hand, data transmission may be completely jointly by different channels, and the packet loss ratio of each channel may be different, in which case, hierarchical processing usually needs to be performed for the data, wherein the important part in the data is transmitted in a stable channel with a relatively low packet loss ratio, while the less important part of the data is transmitted in a non-stable channel with a relatively high packet loss ratio, thereby ensuring that only relative decrease of tone quality, rather than complete failure of decoding of a frame of data, occurs when the packets are lost in the non-stable channel. On the other hand, the bandwidths of certain communication networks (such as the Internet) is not stable, and the bandwidths of different users are different, so the requirements of users with different bandwidths cannot be met simultaneously by using a fixed code rate, while the hierarchical coding scheme can enable different users to obtain the optimal tone quality enjoyment under the bandwidth condition owned by themselves.
In the conventional hierarchical audio coding scheme, for example the standard G.729.1 and G.VBR of ITU (International Telecommunication Union), the core layer uses audio coding, while the extended layer adopts time domain or transform domain coding. The coding scheme of the core layer is independent of that of the extended layer, which will result in a low coding efficiency and also influence the quality of music signal coding in a transmission condition of medium/lower code rate. In other hierarchical schemes, for example, the coding scheme of BSAC (Bit-Slice Arithmetic Code) variable (or retractable) code rate adopted by MPEG4 (Moving Picture Experts Group 4), bit allocation and hierarchical operation are only implemented based on the size of the datum itself without considering the residual perception distribution characteristics of the whole signal per se, so the existing information of the core layer cannot be utilized to help the extended layer to perform coding, thus resulting in a relatively low coding efficiency.